Propeller control with pitch lock



u 9 D. w. MILLER ET AL 2,743,877

PROPELLER CONTROL WITH PITCH LOCK Filed March 15, 1952 CA'Lw/v C. Cows-er Mama DEM/00 DAM: 14 MILLEB OAK/e564 Eoxae M Maw/g4;

77/5/42 Arroems-Ys United States Patent PROPELLER CONTROL WITH PITCH LOCK Dale W. Miller, Brookville, Calvin C. Covert and Darrell E. Royer, Vandalia, and Michael Dernido, Troy, Ohio, assignors to General Motors Corporation, Detroit, Mich., a corporation of Delaware Application March 15, 1952, Serial No. 276,836

13 Claims. (Cl. 170-16021) The present invention relates to a control system for variable pitch propellers and more particularly to a fluid pressure system for controlling the movement of propeller blades.

It is well known that the blades of some variable pitch propellers have an inherent tendency to turn about their longitudinal axes towards a lesser pitch position, due to aerodynamic and centrifugal twisting moments, when the aircraft, on which the propellers are installed, is in flight. in the type of variable pitch propeller wherein a fluid pressure system controls the pitch position of the blades, a failure in the pressure system may result in the turning of the blades to a dangerously low pitch position. To prevent the occurrence of such a situation, the fluid pressure system has been provided with means whereby the propeller pitch will be locked and the propeller will operate as a fixed pitch propeller upon failure of the fluid pressure system. However, when the aircraft has safely landed, it is often desirable to alter the pitch position at which the propeller blades are locked by an auxiliary source of fluid pressure. Accordingly, one of our objects is to provide a fluid pressure control system having pitch locking mechanism with control means for the pitch locking mechanism that enables the pitch position of the blades to be altered by fluid pressure from an auxiliary source when the propeller is not rotating.

The aforementioned and other objects are accomplished in the present invention by providing a fluid pressure system with a pitch lock valve that prevents movement of the blades to a lesser pitch position by establishing a hydraulic-fluid lock in a servo motor utilized to effect movement of the blades when the fluid pressure system fails. In addition, a pitch lock pilot valve, having means allowing opening movement of the pitch lock valve by fluid under pressure from an auxiliary source when the propeller is not rotating, is provided. Specifically, the fluid pressure system of the present invention includes a system pump that supplies fluid under pressure to a high pressure trunk line. The high pressure trunk line is connected to a pressure control valve assembly, a distributor valve, a feathering pump control valve and the pitch lock pilot valve. The pressure control valve regulates the pressure in the system in accordance with the demand for fluid pressure. The distributor valve applies fluid under pressure from the trunk line to the servo motor for controlling the pitch position of the propeller blades. The feathering pump control valve controls the connection of the output of an electric motor-operated feathering pump to the high pressure trunk line when the pressure in the trunk line is less than the pressure output of the feathering pump. The feathering pump serves as the auxiliary source of fluid pressure for opening the pitch lock valve through the pitch lock pilot valve when the propeller is not rotating. Thus, pitch changing movements of the propeller may be elfected even though the system pump has failed.

Further objects and advantages of the present invention will be apparent from the following description, reference being had to the accompanying drawing which is a schematic view of the fluid pressure system.

Referring more particularly to the drawing, system pump 1 operates continuously during propeller rotation to supply hydraulic fluid under pressure to trunk line 10. The pump 1 has an intake line 2 and discharges fluid under pressure through a check valve 3 to the trunk line 10. The check valve 3 prevents fluid flow from line 10 back through the pump 1. Trunk line 10 communicates through passage 4 with a pressure control valve assembly 5. High pressure trunk line 10 also communicates with a feathering pump control valve 6, a pitch lock pilot valve 7, and a distributor valve 8. The component parts of the fluid system are mounted in a regulator of the type shown in the Blanchard et al. Patents Nos. 2,307,101 and 2,307,102.

The distributor valve 8 is connected by lines 12, and 14 to a servomotor 9 which is provided with means for adjusting propeller pitch. A pitch lock valve 11 is positioned between lines 12 and 120 that connect the distributor valve 8 to an increase pitch chamber 13 of the servomotor. The servomotor 9 comprises a cylinder 15 within which a piston 17 is mounted for reciprocal movement. The piston 17 divides the cylinder 15 into the increase pitch chamber 13 and a decrease pitch chamber 19. The piston is connected by means of a rod 21 shown diagrammatically as having a rack 23 formed on one end thereof. The rack 23 engages a pinion 25 which. is rigidly attached to a propeller blade 27 that rotates about a shaft 29. The shaft 29 is driven by an engine, not shown, and rotates in the direction of the arrow.

The pitch lock valve 11 is generally of similar construction to the valve disclosed in .copending application,

Serial No. 263,416, filed December 26, 1951, in the name of Berninger et al. The pitch lock valve 11 comprises a casing 31 provided with a bore 33 having end portions of the same diameter and an intermediate portion of reduced diameter. Disposed within bore 33 and mounted for reciprocal movement therein is a plunger 35. The plunger 35 is provided at opposite ends with pistons 37 and 39 respectively. Piston 37 is provided with an annular circumferential groove within which a rubber O-ring seal 41 is situated. Disposed in one end of the bore 33 is a member 43 provided with a cup-shaped recess 45 of substantially the same diameter as the diameter of piston 37. The member 43 is normally urged by means of a spring 47 into contact with a shoulder 49 which forms the reduced diameter portion of bore 33. In addition, the cup-shaped portion of member 43 is provided with a hole 51 that provides relief for fluid pressure trapped between the cup-shaped opening 45' and the piston 37. 7

Between the pistons 37 and 39, the plunger 35 is provided with a passage 53 that connects the larger diameter portions of bore 33 when the pitch lock valve is in the open position. The piston 39 divides the other end of bore 33 into two chambers 55 and 57. Chamber 55' is connected to line 12 from the distributor valve 8. Chamber 57 is connected by line 16 to an outlet port 18 of the pitch lock pilot valve 7. Chamber 57 is a servo chamber to which fluid pressure is applied for moving the pitch lock valve plunger 35 to the open position. Spring 47 moves the plunger 35 and closes the pitch lock valve when there is no fluid pressure applied to servo chamber 57. Application of fluid pressure to servo chamber 57 of the pitch lock valve is controlled by the pitch lock pilot valve 7.

The pitch lock pilot valve 7 comprises a casing 20 having a bore 22 of constant diameter. Within the bore 22 is disposed a plunger 24 having spaced lands 26, 28 and 30. Plunger 24 is mounted for reciprocal movement within the bore 22, and is normally urged by means of spring 32 to the position it is shown in the drawing. Casing 20 is provided with a pair of pressure inlet ports 34 and 36; a pair of exhaust ports 38 and 40; and the outlet port 18. Inlet port 34 is connected by line 42 to a port 44 in the pressure control valve assembly 5. Inlet portion 36 is connected to high pressure trunk line 10. The annular channel, between lands 28 and 30 of the plunger 24, is connected by means of an axial passage 46 to the portion of bore 22 in which the spring 32 is disposed. Inlet port 36 communicates with the annular channel between lands 26 and 28. The plunger 24 is further provided with stop members 48 and 50 which determine the limits of plunger movement within the bore 22.

Fluid under pressure is constantly supplied from the pump 1 through the line 4 to the pressure control valve assembly or pressure regulating means 5. The pressure control valve assembly comprises an equal area valve 52, a pressure relief valve 54, and a shuttle valve 56. The pressure control valve assembly is generally of similar construction to that disclosed in copending application, Serial No. 37,624, now Patent Number 2,626,669, filed July 8, 1948, in the name of Moore et al. The equal area valve 52 determines the pump operated pressure by demanding from the pump 1 a pressure equal to that re quircd to move the servomotor piston 17 plus a constant additional pressure suflicient to afford the maximum rate of pitch change. One side of the equal area piston 58 is connected through the shuttle valve 56 to the highest pressure present in the servomotor cylinder 15. An opposite side of the piston 58 is exposed to pump pressure through line 4 and passage 60. The piston 58 of the equal area valve 52 is also acted upon by a spring 62 and the thrust of centrifugal force which acts in a direction of the arrow 64, on all valves in the fluid circuit. Accordingly, the equal area valve demands a pressure equal to the highest pressure existent in the servomotor 9 plus a pressure equivalent to the load of spring 62 and the thrust of centrifugal force due to propeller rotation.

The pressure relief valve 54 merely limits the maximum pressure in the fluid pressure system. The shuttle valve 56 is connected by lines 68 and 70 to lines 14 and 12, respectively. Accordingly, whichever of the lines 12 or 14 manifest the greatest pressure, the shuttle valve 56 will communicate this pressure by means of passage 72 to one side of the equal area valve piston 58. In addition, the fluid pressure existent in line 70 will be communicated by passage 74 to one side of the pressure relief valve piston 76. As line 70 is connected to line 12, which is in turn connected through the pitch lock valve 11 to the increase pitch chamber 13 of the servomotor, the pressure relief valve 54 will allow a greater pressure to exist in trunk line when fluid pressure is being supplied to the increase pitch chamber than it will allow when fluid pressure is being supplied to the decrease pitch chamber of the servomotor 9.

The distributor valve 8 comprises a valve guide 78 provided with a pressure supply port 80; a pair of pressure distributing ports 82 and 84 and a drain port 86. Mounted for reciprocal movement in the bore of valve guide 78 is a plunger 88 having spaced lands 90, 92, 94 and 96. Pressure supply port 80 communicates with the annular channel between lands 90 and 92. In addition, the plunger 88 is provided with an axial passage 98 which connects the annular channel between lands 90 and 92 with the annular channel between lands 94 and 96. The drain port 86 communicates with the annular channel between lands 94 and 96 and is connected by means of line 100 to the inlet side of an electric motor-operated feathering pump 59. One end of the plunger 88 is pivotally connected at 102 to a lever 104 supported on a movable fulcrum 106. Between the pivotal connection 102 and the other end of lever 104, a spring 108 is disposed. As the plunger 88 is acted upon by centrifugal force in the direction of arrow 64 during propeller rotation, the plunger 88 will assume an equilibrium position as determined by the opposing thrusts of centrifugal force and the spring 108 acting on the lever 104. Thus when the fulcrum 1.06 is in the position as shown in the drawing, the distributor valve 8 acts as a governor valve in a manner similar to that disclosed in the Blanchard et a]. patents, previously referred to. The movable fulcrum is connected by a mechanical linkage 110 to a control lever 112 which is mounted in the cockpit of the aircraft. By movement of the lever 112 about its pivot 114, the pilot may select the speed at which the propeller will be operated, and if the fulcrum 106 is moved to the right, as viewed in the drawing, so that it is on the other side of spring 108, the spring 108, acting on the lever 104, will move the plunger 88 upwardly. When the plunger 88 is moved upwardly, trunl; line pressure from port is communicated to port 84 and line 12 and line 14 is exposed to drain through passage 98 and port 86. In this manner, the pilot may move the blade 27 to a feathered position. Conversely, the pilot may move the fulcrum 106 to the left, as viewed in the drawing. In this instance, the spring will overcome the thrust of centrifugal force and move the plunger 88 downwardly so that the blade 27 may be moved into the negative thrust range.

If the distributor valve plunger 88 is moved upwardly, as viewed in the drawing, by means of manipulation of the lever 112 or under the thrust of centrifugal force, line 12 will be connected to the high pressure trunk line 10. However, the pressure existent in line 12 will not be communicated to the increase pitch chamber 13 of the servomotor unless the pitch lock valve 11. is in the open position, allowing communication between chamber 55 and line 120. Likewise, if the distributor valve plunger 88 is moved downwardly for any reason, increase pitch chamber 13 will not be connected to drain through lines 120, 12 and unless the pitch loci: valve 11 is in the open position. The existence of fluid pressure in chamber 55 of the pitch lock valve is unable to move the plunge 35 to the position it is shown in the drawing by reason of the area of piston 39 being greater than the area of piston 37. Accordingly, to maintain the pitch lock valve in the position it is shown in the drawing, fluid pressure must be communicated to servo chamber 57. Fluid pressure can only be supplied to servo chamber 57 through line 16 from port 18 of the pitch lock pilot valve 7.

During normal operation of the fluid pressure system. the equal area valve 52 will normally divert excess fluid flow through passage 116 and into passage 118. This excess or surplus flow is communicated by means of port 44 and line 42 to inlet port 34 of the pitch lock pilot valve. This excess fluid flow will act on surface 122 of land 30 and assist the spring 32 in maintaining the plunger 24 in the position it is shown in the drawing. The combined forces of the spring 32 and the excess fluid flow from the equal area valve 52 are opposed by the thrust of centrifugal force in the direction of arrow 64 when the propeller is rotating. Consequently, if no flow is diverted through either passages 116 or 124 from the equal area valve 52 and the pressure relief valve 54, respectively, the thrust of centrifugal force will overcome the thrust of the spring 32 and the plunger 24 will move upwardly, thereby blocking communication between port 36 and port 18. This phenomenon will only occur if the system pump 1 fails to develop sufficient pressure for normal operation of the servomotor 9. However, if this condition should prevail, servo chamber 57 of the pitch lock pilot valve will be connected to drain through line 16, port 18 and port 40 of the pitch lock pilot valve,

amass? and the spring 47 will move the plunger 35 so that communication between chamber 55 and'line 120 is blocked by the piston 37. In this instance, fluid pressure cannot be supplied to or drained from the increase pitch chamber 13 of the servomotor 9 and a hydraulic-fluid lock is established in the servomotor. When a fluid-lock is established in the servomotor 9, the pitch of the blade 27 remains fixed until such time as the pump 1 develops suflicient pressure to establish an excess flow from the equal area valve.

When the fluid pressure system, including the pump 1 and the pressure control valve assembly 5, is operating normally, the excess flow from the pressure control valve will maintain ports 36 and 18 of the pitch lock pilot valve in communication. Accordingly, high pressure from trunk line will be supplied through line 16 to the servo chamber 57 and the pitch lock valve will be maintained in the open position. With the pitch lock valve in the open position, fluid pressure can be supplied to or drained from either the decrease or increase pitch chambers of the servomotor under the control of the distributor valve 8.

If the pump 1 should fail, the propeller pitch will remain fixed at the pitch position the blades were in when the malfunction occurred. This will ordinarily enable the pilot to maintain the aircraft in flight and also enable the pilot to make a safe landing. However, when the craft is safely on the ground, it is often desirable to alter the pitch position of the blades. To accomplish this result, the present invention provides an auxiliary source of fluid pressure developed by the feathering pump 59. The feathering pump 59 develops a pressure output of approximately 500 p. s. i. and is operated by an electric motor. The electric motor is energized by wires 61 and 63 from a 28 volt D. C. source of power. Line 63 is provided with a switch 65 which may be manually actuated by the pilot. However, the output of the pump 59 will not be supplied to trunk line 10 unless the fluid pressure in trunk line 10 is less than the fluid pressure developed by the pump 59. The connection of the pump 59 to the trunk line 10 is controlled by the feathering pump control valve 6 which comprises a check valve 67 and a pressure control valve 69. High pressure trunk line 10 communicates through passage 71 with one side of the check valve 67 and the outlet of pump 59 communicates through passage 73 with the other side of the check valve. Thus, if the pressure in line 10 is greater than the pressure in passage 73, fluid pressure developed by the pump 59 will be diverted through passage 75 to a port 77 of the pressure control valve 69. The pressure control valve 69 comprises a plunger 79 having spaced lands 81 and 83. The end surface of land 83 is exposed to pressure existent in trunk line 10 through a passage 85. If the pressure in passage 85 is greater than about 450 p. s. i., the plunger 79 Will allow communication between the ports 77 and 87, allowing flow from the pump 59 to recirculate through line 89 to the inlet side 91 of the feathering pump. The inlet side 91 of the feathering pump is also connected by means of line 93 to a feathering pump inlet valve assembly 95 which always assures a supply of fluid to the inlet side 91 of the pump 59. However, if the pressure in trunk line 10 falls below 450 p. s. i., a spring 97 will move the plunger 79 to the position shown in the drawing, and the check valve 67 will open, thereby porting the output of pump 59 into the high pressure trunk line 10.

If the propeller is not rotating and the feathering pump control valve 6 has ported the output of pump 59 into the trunk line 10, the spring 32 will have moved the plunger 24 of the pitch lock valve to the position it is shown in the drawing. In this instance, fluid pressure will be supplied to pitch lock valve servo chamber 57 and the pitch lock valve will be maintained in the open position. With the pitch lock valve in the open position, the pitch position of the blade 27 may be controlled by the distributor valve 8. However, if the propeller is rotating, the pitch lock valve will not be open by reason of the fact that the thrust of centrifugal force will move the pitch lock pilot valve plunger 24 upwardly. As the pressure output of the auxiliary source of fluid pressure pump 59 is insufficient to cause the pressure control valve assembly 5 to divert any flow through line 42, the pitch lock pilot valve will block this connection between ports 36 and 18 until such time as the spring 32 overcomes the thrust of centrifugal force acting on the plunger 24. This ordinarily will not occur until a propeller is substantially stationary.

When the pressure system is operating normally, the diverted flow from the pressure control valve assembly 5, which passes through line 42 and port 34 to the pitch lock pilot valve, will flow to drain through passage 46 and port 38, and also through port 38 directly. It is apparent from the aforegoing description that, by providing the pitch lock pilot valve and an auxiliary source of fluid pressure, a more complete control system is aiforded. Moreover, by the inclusion of the pitch lock valve, a fixed propeller pitch will be maintained if the system pump 1 fails while the propeller is rotating. Accordingly, the aerodynamic and centrifugal twisting moments acting upon the blades will be unable to move the blades to a lesser pitch position. However, when the aircraft has safely landed, the pitch position of the blades may be varied by fluid pressure from the auxiliary source after propeller rotation has substantially ceased.

While the embodiment of the present invention as herein disclosed, constitutes a preferred form, it is to be understood that other forms might be adopted.

What is claimed is as follows:

1. In a control system for a variable pitch propeller, the combination including, a fluid motor for adjusting propeller pitch, a fluid pressure system for actuating said motor, said system comprising, a system source of fluid pressure, an auxiliary source of fluid pressure connectable to said system source, a valve connected to said system source and said motor for controlling fluid flow to and from said motor, and means controlling operation of said auxiliary source, pitch lock means operatively connected with said motor for maintaining a fixed propeller pitch upon failure of said system source by trapping fluid within said motor, and means including a centrifugally responsive device in said fluid pressure system and operatively connected with the pitch lock means for rendering said pitch locking means ineffective when said auxiliary source is operative and the propeller is stationary.

2. In a control system for a variable pitch propeller, the combination including, a fluid pressure operated servomotor operatively connected to said propeller for adjusting the pitch position thereof, a fluid pressure system for actuating said servomotor, servo actuated means operatively connected to said servomotor for maintaining a fixed propeller pitch upon failure of said fluid pressure system by trapping fluid within said servomotor, and means including a centrifugally responsive device in said fluid pressure system and operatively connected with said servo actuated means for rendering said servo actuated means ineffective when the propeller is stationary.

3. The combination set forth in claim 2 wherein said fluid pressure system includes a source of fluid pressure continuously operable during propeller rotation, a high pressure trunk line connected to said source, pressure regulating means connected to said trunk line for controlling the pressure therein, valve means connected to said trunk line and to said servomotor for controlling fluid flow to and from said servomotor, a selectively operable auxiliary source of fluid pressure, and valve means for connecting said auxiliary source with said trunk line.

4. Control mechanism for a variable pitch propeller including, in combination, a fluid pressure operated motor for adjusting the pitch position of said propeller, a

system source of fluid pressure, a trunk line connected to said system source, pressure regulating means connected to said trunk line for controlling the pressure therein. a valve having a supply port connected with said trunk line and a pair of control ports connected with said motor for controlling fluid flow to and from said motor, a pitch lock valve in circuit connection between one of said control ports and said motor, a pitch lock pilot valve having a supply port connected with said trunk line and a distribution port connected with said pitch lock valve, a. selectively operable auxiliary source of fluid pressure, means connecting said auxiliary source with said trunk line when the pressure in said trunk line is below a predetermined potential, said pitch lock valve effecting a fluid lock in said motor upon failure of said system source whereby the propeller pitch is fixed, said pitch lock pilot valve controlling the position of said pitch lock valve whereby said fluid lock may be rendered inoperative when said auxiliary source is operative.

5. The combination set forth in claim 2 wherein said pressure regulating means includes an equal area valve,

and a passage associated with said equal area valve receives fluid flow diverted by said equal area valve.

6. The combination set forth in claim 3 wherein said pitch loci; pilot valve includes a servo chamber connected with the passage in said pressure regulating means for receiving said diverted fluid flow, said diverted fluid flow maintaining said pitch lock pilot valve in the open position.

7. The combination set forth in claim 2 wherein the pitch lock valve is servo actuated, and wherein said pitch lock pilot valve controls the application of fluid pressure from said trunk line through said distribution port to the servo actuated pitch lock valve.

8. Control mechanism for a variable pitch propeller having adjustable blades and having fluid pressure operated motor means for adjusting the pitch position of said blades, said blades being subject to external aerodynamic and centrifugal twisting moments which tend to move said blades to a predetermined pitch position, including in combination, means operable to produce a continuous system supply of fluid pressure during rotation of said propeller, means operable to produce an auxiliary supply of fluid pressure when said propeller is stationary, a valve interconnecting said system supply and opposite sides of said motor means for controlling the application of pressure fluid to opposite sides thereof, means in too interconnection between said valve and one side of said motor means for trapping fluid within said one motor means side to maintain a fixed propeller pitch upon failure of said system supply, the application of pressure fluid to said one meter means side actuating said motor so as to move said blades away from said predetermined pitch position, and means including a centrifugally responsive device operatively connected to said fluid trapping means aud operable to release the trapped fluid in said one motor means side when the propeller is stationary and the auxiliary supply is operative.

9. In a control system for a variable pitch propeller, the combination including, a fluid pressure operated servomotor opcrativcly connected to said propeller for adjusting the pitch position thereof, a fluid pressure system for actuating said servomotor, said system including a source of fluid pressure continuously operable during propeller rotation, a high pressure trunk line connected to said source, pressure regulating means connected to said trunk line for controlling the pressure potential. therein, valve means connected to said trunk line and to said servomotor for controlling fluid flow to and from said servomotor, a selectively operable auxiliary source of fluid pressure, and valve means for connecting said auxiliary source with said trunk line, said pressure regulating means comprising an equal area valve, :1 pressure relief valve, and a passage for receiving diverted flow from these valves, servo-actuated means operatively connected to said servomotor for maintaining a fixed propeller pitch upon failure of said fluid pressure system by trapping fluid within said servomotor, means in said fluid pressure system and operatively connected with said servo actuated means for rendering said servo actuated means ineffective, and passage means for conducting diverted flow from said pressure regulating means to said means for rendering said servo actuated means ineffective.

10. In a control system for a variable pitch propeller, the combination including, a fluid pressure operated servomotor operatively connected to said propeller for adjusting the pitch position thereof, a fluid pressure systern for actuating said servomotor, pitch lock means operatively connected to said servomotor for maintaining a fixed propeller pitch upon failure of said fluid pressure system by trapping fluid within said servomotor, and means including a ccntrifugally responsive device in said fluid pressure system and operatively connected with said pitch lock means for rendering said pitch lock means ineffective when the propeller is stationary and the fluid pressure system is operative, said pitch lock means comprising a valve in circuit connection between said fluid pressure system and said servomotor, said valve including a plunger having pistons at opposite ends thereof, one of said pistons controlling the connection between said fluid pressure system and said servomotor, and the other of said pistons being subjected to system fluid pressure for effecting movement of said plunger.

11. In a control system for a variable pitch propeller, the combination including, a fluid pressure operated servomotor operatively connected to said propeller for adjusting the pitch position thereof, a fluid pressure system for actuating said servomotor, servo actuated means opcrativcly connected to said servomotor for maintaining a fixed propeller pitch upon failure of said fluid pressure system by trapping fluid within said servomotor, and means in said fluid pressure system and operatively connected with said servo actuated means for rendering said servo actuated means ineffective, said last recited means comprising, a. valve including a casing having a bore in which a plunger is disposed, resilient means operatively associated with said plunger and tending to maintain said valve open, the thrust of said resilient means being opposed by the thrust of said centrifugal force acting on said plunger when the propeller is rotating, and servo actuated means for assisting said resilient means so as to maintain the valve in the open position when the propeller is rotating, said last recited servo actuated means being connected to said fluid pressure system whereby when the fluid pressure system fails, the valve will be closed under the urge of centrifugal force when the propeller is rotating.

12. In a control system for a variable pitch propeller, the combination including, a fluid motor for adjusting propeller pitch, a fluid pressure system for actuating said motor, said system comprising a source of fluid pressure and a valve connected to said source and to said motor for controlling fluid flow to and from said motor, means operatively connected with said motor for trapping fluid therein to maintain a fixed propeller pitch upon failure of said fluid pressure system, a servo for deactivating said fluid trapping means, and means including a contrifugally responsive device in said fluid pressure system and operatively connected to said servo for applying pressure fluid to said servo to prevent the trapping of fluid within said motor when the fluid pressure system is operative and the propeller is stationary.

13. In a control system for a variable pitch propeller, the combination including, a fluid motor for adjusting propeller pitch, a fluid pressure system for actuating said motor, said system comprising a source of fluid pressure, pressure regulating means operatively connected to said source for controlling the pressure potential thereof, and a valve connected to said source and to said motor for controlling fluid flow to and from said motor, means operatively connected to said motor for trapping fluid 10- therein to maintain a fixed propeller pitch upon failure References Cited in the file of this patent of said fluid pressure system, a servo for deactivating UNITED STATES PATENTS said fluid trapping means, and means including a cen- 2320195 RindfleiSch Ma 25 1943 trifugally responsive device in said fluid pressure system 41 6 Keller 1944 and operatively connected to said servo for applying pres- 5 2:413:439 Drake a 1946 sure fluid to said servo to prevent the trapping of fluid within said motor when the fluid pressure system is op- FOREIGN PATENTS erative and the propeller is stationary. 562,845 Great Britain July 19, 1944 

